1. Field of the Invention
The present invention relates to a production process for high purity polycrystal silicon which is a raw material of silicon for semiconductors and silicon for solar batteries and a production apparatus for the same.
2. Description of the Related Art
Polycrystal silicon is used as a raw material of silicon for semiconductors and as a raw material of silicon for solar batteries. In the state that popularization of solar batteries is expanded to a large extent particularly in recent years, demand to polycrystal silicon which is a raw material therefor is increased as well.
However, the existing state is that a crucible residue obtained after pulling up single crystal silicon for semiconductors and scrapped products such as cutting scraps of a single crystal silicon ingot are used as polycrystal silicon which is a raw material of silicon for solar batteries. Accordingly, polycrystal silicon used for solar batteries depends on the movement of the semiconductor industry in terms of both quality and amount, and as a result thereof, it stays in the situation that it is chronically short.
In this connection, a representative production process for high purity polycrystal silicon which is a raw material of single crystal silicon for semiconductors includes a Siemens process. In this Siemens process, high purity polycrystal silicon is obtained by hydrogen reduction of trichlorosilane (HSiCl3) (refer to, for example, U.S. Pat. No. 2,867,306).
In a conventional Siemens process, seed bars 50 of silicon are put, as shown in a production apparatus 60 in FIG. 6, in a reactor 30 of a water-cooled bell jar type; an electric current is applied through the above seed bars 50 of silicon to heat the seed bars 50 to about 1000° C.; trichlorosilane (HSiCl3) and hydrogen (H2) of a reducing agent are introduced into the reactor 30 from a lower part to reduce silicon chloride; and resulting silicon is adhered selectively onto the surfaces of the seed bars 50, whereby bar-like polycrystal silicon is obtained. The above Siemens process has, in addition to the advantage that the raw material gas is vaporized at a relatively low temperature, the advantage in terms of an apparatus that the atmosphere is readily sealed since the reactor 30 itself is cooled with water, and therefore it has so far been widely popularized and employed.
In the Siemens process, however, the seed bars 50 is allowed to generate heat by applying an electric current, and therefore as bar-like silicon grows by adhesion of polycrystal silicon to allow an electric resistance to be slowly reduced, excessive electricity for heating has to be applied. Accordingly, the growth limit is present because of a balance with the energy cost, and involved therein are the problems that the production efficiency is inferior since the production facility is operated by a batch system and that an electric power consumption rate accounts for a large proportion in the price of polycrystal silicon which is the product.
Further, the seed bars 50 require specific facilities such as a dedicated reaction apparatus, a single crystal pulling-up apparatus and a cutting apparatus and specific techniques therefor in producing, and therefore the seed bars 50 themselves have become expensive.
Production processes for polycrystal silicon other than the Siemens process includes, for example, processes in which it is produced by reduction of silicon tetrachloride (SiCl4) using metal reducing agents (refer to, for example, Japanese Patent Application Laid-Open No. 34519/2003 and Japanese Patent Application Laid-Open No. 342016/2003). To be specific, it is a process in which gases of silicon tetrachloride and zinc (Zn) are supplied into a lateral reactor made of quartz which is heated at about 1000° C., whereby polycrystal silicon is allowed to grow in the reactor.
Supposing that in the process described above, by-produced zinc chloride (ZnCl2) is separated into zinc and chlorine by a method such as electrolysis to use again resulting zinc as a reducing agent and that resulting chlorine is reacted with inexpensive metal silicon to thereby synthesize silicon tetrachloride, which can be used as the raw material gas, a process of a recycling system is constituted, and therefore involved therein is the possibility that polycrystal silicon can be produced at a low cost.
In the above process, however, the polycrystal silicon obtained by the reaction grows from the wall of the reactor and therefore is liable to be exerted by an effect of contamination from the material of the reactor. Further, the above quartz-made lateral reactor has involved the problem that the polycrystal silicon is inferior in a production efficiency in addition to the problem that the reactor itself is broken due to a difference in a thermal expansion coefficient from that of the polycrystal silicon.